The amount of information transmitted through an optical network has increased dramatically. Further, a configuration in which a path that is to be established in an optical network can be flexibly changed has been required. Thus, a method for processing, between an optical transmitter and an optical receiver, an optical signal that is transmitted from the optical transmitter so as to improve performance and/or a function of an optical network has been proposed.
In order to mitigate a waveform distortion due to nonlinear effects in an optical fiber, a method for remodulating an optical signal so as to make a transmission distance longer has been proposed (see, for example, Documents 1 and 2 below). A method for RZ-pulsing an optical signal with a clock signal synchronized with a data signal so as to make a transmission distance longer has been proposed (see, for example, Japanese Laid-open Patent Publication No. 2009-60461 (Japanese Patent No. 5088052)). A method for performing a frequency shift for one or more optical signals in a WDM optical signal by optical remodulation so as to decrease fragmentation of a spectrum and to improve frequency utilization efficiency has been proposed (see, for example, Documents 3 and 4 below). A method for superimposing, for example, by frequency modulation, a supervisory signal on an optical signal that transmits a main signal has been proposed (see, for example, Japanese Laid-open Patent Publication No. 2012-120010, U.S. Patent Application Publication No. 2010/0014874 (U.S. Pat. No. 7,957,653), and Document 5 below).    Document 1: Benjamin Foo et al., Optoelectronic method for distributed compensation of XPM in long haul WDM systems, OFC 2015, Th2A, 24    Document 2: Benjamin Foo et al., Optoelectronic method for inline compensation of XPM in long-haul optical links, Optics Express, Vol. 23, issue 2, pp. 859-872 (2015)    Document 3: Satoshi Shimizu et al., Demonstration of Multi-hop Optical Add-Drop Network with High Frequency Granular Optical Channel Defragmentation Nodes, OFC 2015, M21.4    Document 4: Feng Tian et al., Generation of 50 Stable Frequency-Locked Optical Carriers for Tb/s Multicarrier Optical Transmission Using a Recirculating Frequency Shifter, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 8, Apr. 15, 2011    Document 5: Tetsuya Kawanishi et al., High-Speed Optical FSK Modulator for Optical Packet Labeling, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005
The polarization state of an optical signal varies when it is transmitted through an optical fiber cable. On the other hand, an optical modulator has a polarization dependence. Thus, in order to improve modulation efficiency of the optical modulator, it is preferable that the polarization state of an optical signal input to the optical modulator is controlled properly. Accordingly, there exist problems below in a configuration in which an optical signal is remodulated between an optical transmitter and an optical receiver.    (1) The cost is increased if a polarization controller is provided at an input side of the optical modulator.    (2) A method for processing a polarization multiplexed optical signal is not obvious.    (3) It is difficult to collectively process a plurality of optical signals when not all of the polarization states of the optical signals in a wavelength division multiplexed optical signal are the same.
Further, in a configuration in which a supervisory signal is superimposed, for example, by frequency modulation, on an optical signal that transmits a main signal, a method for processing the supervisory signal without converting the optical signal into an electric signal is unknown.
As described above, there exist problems to be solved in the method for remodulating an optical signal that is propagated in an optical network.